This extension is the best thing to happen to Firefox since Electrolysis and Quantum. I use it on, and it works well on, nearly every Web site. For the few sites that don't jibe with default settings, it manages per-site settings for custom colors or flat out disablement very elegantly. I can't give this enough stars.
UPDATE: I decided that this is the first browser extension worthy of giving the developer a donation for. And I did that. And I suppose I couldn't resist tooting my own horn about it. *toot*

Saved my eyes for sure.

Only used for a day, but thus far it's perfect. Love the color customization, it's not in-depth but it's just enough to set it somewhere comfortable for your eyes. It's also smart enough to correct embedded white backdrops, unlike the other version I tried. Awesome!

Tuts wieder, klasse.

GREAT

Edit: Loading issues seem to have been resolved!

I'm also having issues with the latest update breaking page loading. Hoping this gets the dev's attention and will happily revise to 5 stars when resolved!

this extension solves a surprisingly hard problem surprisingly well. It just works.